The sputtering technique is a well-established physical vapor deposition (PVD) method, which is used for depositing thin films for a broad variety of applications. In particular, wear protective coatings for cutting tools applications can be successfully deposited by using the sputtering technology. Especially, the coating quality can be improved significantly by using high-power impulse magnetron sputtering (HIPIMS) methods.
A fundamental part of a sputtering apparatus is the at least one sputtering target used as material source for providing, the necessary deposition species for the coating growth on the substrate surface placed in the coating chamber. In the case of reactive sputtering processes the species obtained from the target react with the reactive gas present in the coating chamber for forming the film on the substrate surface to be coated. Target material from the target surface is sputtered b collision of ions (essentially ions from an ionized non-reactive gas), which are attracted to the target surface through the application of a magnetic field. Thus, the sputtering process at the target leads to erosion of the target surface and consequently to a change of the target weight.
Particularly, strong erosion of the target surface can be observed on the areas of the target subjected to a strong magnetic field, which leads to undesirable changes in the coating plasma conditions and consequently in the coating growth on the substrate surface.
Different methods for avoiding these undesirable changes are proposed currently by the state of the art.
For example, the patent document WO0116394A1 proposes a method for controlling reactive sputter deposition of a compound formed from reaction between a reactive species that results from excitation of a reactive gas and a material included in a target acting as a cathode. The method comprising the steps of: establishing reactive sputter deposition at a nominal flow rate or partial pressure of the reactive gas; and stabilizing the cathode voltage at a nominal voltage by adjusting the power supplied to the cathode, wherein said normal voltage and nominal flow rate or partial pressure are determined according to an equilibrium state or steady state condition among said cathode voltage, said nominal flow rate or partial pressure of the reactive gas, and power supplied to the cathode.
Similarly, the patent documents U.S. Pat. No. 5,783,048 A, WO0005745 and WO2013045493A1 propose different sputtering apparatus and methods for controlling sputter deposition by adjusting the magnetic field in order to compensate for the undesirable changes caused by target erosion during sputtering of the target surface.
U.S. Pat. No. 5,783,048A discloses a sputtering apparatus for forming a thin film on a substrate, the formed film exhibiting a substantially uniform thickness. The apparatus including a target for providing target material for forming the thin film, wherein the target includes a first area. The sputtering apparatus further includes a plasma discharge to enable removal of target material from the target. In addition, a main magnet is provided for generating a main magnetic field for controlling the plasma discharge to remove the target material. Further, a compensating magnet is utilized which is positioned adjacent to the first area. The compensating magnet generates a compensating magnetic field which interacts with the main magnetic field to control the plasma discharge in the first area to form a desired erosion pattern in the first area and enable formation of a substantially uniform film thickness on the substrate.
WO0005745A1 discloses an apparatus and a method for compensating for process-related asymmetries produced in physical vapor processing of a surface, in particular, when sputtering material from a source is deposited onto a substrate for forming a film. A compensating magnet is configured and positioned to produce a compensating magnetic field to offset the effects of chamber and process-related asymmetries, particularly those that affect the distribution of plasma processing on a substrate where the plasma has been otherwise symmetrically produced.
WO2013045493A1 proposes a method for magnetron sputtering in a sputtering apparatus, which has a cathode, a target, which is arranged on a surface of the cathode or is part of said surface of the cathode, and a magnet set arranged on a side of the cathode facing away from said surface, wherein the target is eroded at least in some areas on an eroding surface during the sputtering, and wherein a distance between the magnet set and the eroding surface of the target is adjusted during the course of the eroding in such a way that an impedance of a circuit comprising the cathode changes only by less than a predetermined value due to the eroding during the course of the eroding, which predetermined value is less than an impedance difference between the impedance for the non-eroded target and the impedance for the maximally eroded target at an unadjusted distance.
However, coating different kind of substrates for different applications in the industrial production context involves additional difficulties which are not easy to overcome by using the current technologies provided by the state of the art.
For attaining the necessary reproducibility, operating efficiency and for ensuring product quality in the industrial production context it is essential to operate all coating batches at stable conditions (especially same conditions as possible), but in this context the targets must be operated for a very long time for performing several batches. It leads not only to coating conditions instabilities caused by the art and/or form of the erosion at the target surface, but also caused by a considerable reduction of the target mass resulting from the prolonged erosion of the target after several batches, which in the context of the present invention will be called target age. The target age leads in turn to undesirable changes in the sputtering characteristic of the target and in the film deposition rate, particularly by accomplishing reactive sputtering processes, as it is shown in the FIGS. 1 and 2. For most of the coating applications, and particularly in the industrial production context are these changes unacceptable.
FIG. 1 shows the variation of the source (target) voltage in volts and current density in amperes per square centimeter in relation to the target weight in grams obtained by reactive sputtering, of one aluminum chromium target in an argon-nitrogen atmosphere by applying a constant power density at a target of about 1 kW/cm2 and maintaining nitrogen partial pressure constant at a value of 0.27 Pa. Also the substrate temperature was maintained constant during sputtering for all attempts. A magnetic field was generated by using permanent magnets arranged behind the target in order to increase the probability of detention of electrons in the area in front of the target. The used target was a disk-shaped target having constant diameter ∅150 mm and variable thickness which was reducing during sputtering causing thus reduction of the target weight. The element composition of the used target was 60 at.-% Al and 40 at.-% Cr.
FIG. 2 shows the measured coating thickness in μm of the films deposited on the surface of flat samples in relation to the weight in grams of the target used for the film deposition. Each film was deposited by reactive sputtering of one AlCr-target in an argon-nitrogen atmosphere. The films were deposited by using the same coating configuration and under same coating conditions as those used for accomplishing the experiments displayed in FIG. 1. In order to be able to analyze the relation between deposition rate and target weight, each film was deposited on the corresponding samples during 117 minutes under same coating conditions regarding power density at the target, reactive partial pressure, substrate temperature. Only the target weight was different by accomplishing the deposition of each film.